As the number of devices that share a telephone line at a business or residence location increases, maintaining compatibility between them frequently becomes a problem. For example, telephone answering devices and facsimile machines cannot normally share the same telephone line because one or the other will dominate incoming calls by being the first to respond to a ringing signal. Consequently, facsimile machines generally use a separate telephone line which means additional wiring and expense.
U.S. Pat. No. 4,582,956 was issued on Apr. 15, 1986, to Carolyn Doughty and suggests one solution to the problem of selecting a particular automatic answering station when many share a common telephone line. Apparatus at a telephone company central office, such as Automatic Number Identification (ANI) equipment, encodes the telephone number of a calling party and transmits it to each called party that subscribes to a Call Identification service. This service, although growing in acceptance, has not been without controversy. In this service, frequency shift keyed signals carrying the calling party telephone number are transmitted during a silent interval between ringing. This information is displayed to the called party so that the identity of the calling party is known prior to answering the telephone. This information can also be used to select one particular station on the telephone line to respond to the incoming call. However, this service is not available everywhere; a monthly fee must be paid; and the calling party's telephone number does not necessarily reveal the desired receiving station.
Telephone Call Directors provide another solution by answering all incoming calls, identifying the nature of the call, and then interconnecting the incoming call with the proper destination. While such equipment advantageously minimizes the number of incoming lines, it is intrusive and is primarily useful in a business application for reasons of cost, size and wiring layout. Call Directors have found little acceptance in a home environment where, perhaps, only one additional answering device needs to share a common telephone line.
U.S. Pat. No. 4,949,372 was issued on Aug. 14, 1990 to D. C. Steele and discloses a technique for ascertaining whether an incoming call is intended for a local security system terminal which shares a telephone line with other automatic answering equipment. The telephone line is continuously monitored for the presence of a unique tone pair whose presence enables communication with the security system. Although this technique is useful, it is still susceptible to intrusion by unauthorized callers who can easily transmit a pair of audio frequency tones.
In another well-known technique, a central security monitoring facility calls the local security system terminal, but then only allows a small number of rings to occur (frequently just one). Other automatic answering equipment which shares the telephone line is not activated by this small number of rings; but the local security system is now primed to seize the line the next time that ringing commences (if within a very brief time interval). Unfortunately, this technique is readily duplicated by unauthorized persons seeking access to the local security system, and often responds to other calling parties who inadvertently duplicate the sequence.
It is particularly desirable to add security system equipment to the same telephone line at a subscriber's premises having a facsimile machine or a personal computer modem connected thereto. Modern security equipment, being remotely programmable, must interact with a central security monitoring facility so that its status can be verified or changed as the need arises. It is generally unacceptable to wait until the security system is next scheduled to report to its monitoring facility before its programming can be modified or functional tests performed. Accordingly, an arrangement that allows security system equipment to share a telephone line with other automatic answering equipment, without interference, would be of great benefit. Further, the security system must be sufficiently robust to preclude unauthorized persons from accessing the security system over the telephone line.